I have only recently acquired this oleander (since I noticed they grow like weeds here... I am working in the Kurdish region of North Iraq). It seemed to be growing fine on my balcony, but after a recent dust storm, I tried wiping (with a damp cloth) some of the leaves which were coated in dust. After that, all the blossoms started drooping.... and as you can see, now the whole plant is drooping. It is also turning dry and crispy. I don't know whether to water it more, water it less, or give up and start again. PLEASE HELP. (Sorry, I don't know how to rotate my photo)
The same strange thing happened to a topiaried giant verbena I came across in our greenhouses recently. The foliage collapsed over a very hot weekend, and it was quite dusty. It certainly wouldn't have been the same conditions as a duststorm in Iraq and we joked that it might have had heatstroke. Unfortunately the verbena tree did not recover and it is still collapsed, bearing burned foliage on nearly all leaves (brown spots). How is your oleander doing now? I would personally cut off all dried or collapsed foliage, even branches. Drench in water until running out of drainholes, then leave off until dry again. Then water sparingly after dead foliage is cut back. Check the type of soil you have there. It seems a bit heavy for a sandy soil lover? After pruning the dead or damaged branches/foliage, I might also remove it from the pot (when soil is DRY) to inspect roots. Allow the tree up to 1 full month to 2 months to recover completely and put out new shoots. If it puts out new shoots, increase watering and drenching. In the future, try to protect it by bringing it inside during such dust storms. In the future, it might also help to have a large saucer of pebbles and water - humidifier tray - under each tree to prevent the dry air from getting the better of the plant.
It's from the wiping, not the dust or dryness. Oleander, especially in desert conditions, exudes a reflective protective oil on the leaves to prevent overheating. Wiping it off, even though it is dusty, makes the plant more vulnerable to heat. The better solution to dusty Oleander is to hose it down (mimc heavy rain) - this will take the dust off without disturbing the oil. You need to keep it fairly sheltered until it can regenerate that oily coating. And as Flaxe notes, sandy soil is probably better for it.
